The present invention relates in general to satellite transmission and receiving devices and, in particular to an improved audio signal expander apparatus for integration into a satellite receiver device and connectable to the detector output thereof. The present audio signal expander apparatus serves to recover satellite audio signal transmissions which are carried by satellite audio subcarrier services utilizing compressed audio signal transmission processing wherein the expansion ratio imparted upon the audio signal by the expander apparatus varies with both the audio signal amplitude and frequency.
The present audio signal expander apparatus is designed to be compatible with the Panda I companding system which is used almost exclusively in the United States as a means to improve the audio quality on satellite analog subcarrier audio services. The technique of companding refers to the compression of the audio signal prior to its transmission via satellite and the subsequent expansion of the audio signal upon being received at a satellite receiving station.
The technique of companding is necessary and thus utilized in order to compensate for the limited amount of power typically available on a transponder carried by the satellite which transponder transmits the audio information required for television broadcast plus additional signals which may comprise video signals and/or other non-television related signals.
In order to fit more audio subcarrier signals onto a satellite transponder the level and deviation of each audio subcarrier is reduced to conserve power. The reduction of both the level and deviation of an audio subcarrier results in the undesirable reduction of both the signal-to-noise ratio (SNR) and the dynamic range of the recovered audio signal.
The technique of companding serves to improve both the signal-to-noise ratio and the recovered dynamic range of the audio signal by compressing the dynamic range before transmission and, in turn, expanding the dynamic range of the audio signal after reception at the satellite receiving station. As is known by those skilled in the art, Panda I refers to a type of companding which has been optimized for satellite audio transmission.
One prior art device accomplishes the expansion of a received audio signal in a manner which varies as a function of both the signal and input amplitude and input frequency by placing a frequency selective negative feedback circuit across a fixed 2:1 expansion circuit. This prior art device further utilizes a RMS detector circuit which is entirely external to the fixed expander circuit.
The present invention, namely an audio signal expander, is designed to be integrated into a satellite receiver and connected immediately to the detector output of the satellite receiver prior to the audio signal undergoing any level control or deemphasis. A fixed gain block, or other interface circuit may, however, be utilized in the receiving circuit device.
Accordingly it is an object of the present invention to provide an audio signal expander apparatus which is compatible with the Panda I audio signal companding system.
It is further an object of the present invention to expand the dynamic range of the audio signal by an expansion ratio which varies with both the audio signal amplitude and frequency.
It is another object of the present invention to provide an apparatus which is connectable to the detector output of the satellite receiver and which incorporates level control and deemphasis circuitry.
Yet another object of the invention is to provide an audio signal expander apparatus which recovers a compressed audio signal to thereby restore the original dynamic range of the audio signal towards increasing the signal-to-noise ratio of the signal.
An additional object of the present invention is to provide an audio signal expansion apparatus which is based upon a fixed 2:1 expansion circuit which is made variable through the use of a negative feedback circuit connected across the fixed expansion circuit.
It is additionally an object of the present audio signal expansion apparatus to provide for the independent expansion of each channel of a stereo audio signal.
Another object of the invention is to provide an audio signal expander apparatus which utilizes an internal on-chip RMS detector circuit.
The present invention further has as an object the provision of an audio signal expander apparatus which requires fewer components and a single voltage power supply so as to in turn require less power to operate and thus lower operating temperatures providing for increased reliability.
An additional object of the present invention is to provide an audio signal expander apparatus which is less susceptible to external noise, such as that generated by 60 Hz power supply sources.
These and other objects of the invention will become apparent in light of the present specification and drawings.